(a) Field of the Invention
The present invention relates to a photometer equipped with a mechanism for compensating film speed such as ASA speed or DIN speed (a mechanism for compensating quantity of light incident on a photodetector depending on film speed) and so adapted as to prevent a difference in quantity of light depending on directions of incidence caused by a mask having asymmetrical apertures used in said compensation mechanism.
(b) Description of the Prior Art
For a photographic camera having a relatively simple construction, ASA speed is compensated as illustrated in FIG. 1. In this drawing, the reference numeral 1 represents a photodetector, and the reference numeral 2 designates a mask having asymmetrical apertures 2a for limiting quantity of light reaching the photodetector 1. The mechanism for compensating ASA speed consisting of the photodetector 1 and mask 2 functions as described below. So far as an object to be photographed is at the same brightness, it is necessary to select a higher shutter speed for a film having higher ASA speed than that for a film having a lower ASA speed. It is therefore required to adjust the quantity of light incident one the photodetector 1 in accordance with variation of film speed (ASA value). Such adjustment is performed by shortening the time required for total of light quantity to reach a certain predetermined value when the ASA value is high, and prolonging the time when the ASA value is low. Speaking specifically with reference to the film speed compensating mechanism shown in FIG. 1, the adjustment is performed by displacing the photodetector 1 and mask 2 relatively to each other along the chain line 3. The relative positional relationship between the photodetector 1 and mask 2 is set in such a manner that the photodetector 1 is located at the position 1' for a film having a speed of ASA 400, for example, and the position 1" for a film having a speed of ASA 100.
In a case where a mask having apertures of such asymmetrical shapes as those shown in FIG. 1 (called tear drop shapes) is used, the asymmetry makes the quantity of incident light different depending on the direction of incidence of the light, thereby making it impossible to obtain a correct shutter speed. When the photodetector is located at the position of the cross-hatched circle in FIG. 1, a light pencil incident obliquely from the left side has a sectional area which is different from that of a light pencil incident obliquely from the right side. FIG. 2 shows a sectional view taken along the II--II line in FIG. 1. Let us consider a light pencil A and another light pencil B in FIG. 2. In this drawing, the light pencil A has a sectional area in the a--a direction as shown in FIG. 2a, whereas the light pencil B has a sectional area in the b--b direction as shown in FIG. 2b. Therefore, there is a difference in the quantity of light that is incident from the left side versus that incident from the right side, whereby the quantity of light incident on the photodetector is made different between two cases where the left side is brighter and where the right side is brighter. Since the shutter operates at a predetermined speed, the difference in distribution of brightness on the object to be photographed results in underexposure and overexposure.
As is clear from the above descriptions made on the mechanism for compensating ASA speed, general film speed compensating devices having asymmetrical apertures before the photodetector pose the problem of an unbalance in the quantity of light for exposure.